This invention relates to photoresists for use in fabrication of semiconductor devices, and more specifically, to a low temperature process for fully hardening the photoresist.
Photoresists are used in conjunction with many different types of processing steps during semiconductor circuit manufacture. Many of these manufacturing or processing steps require that the photoresists be exposed to temperatures above 150 degrees C., usually 150 to 200 degrees C. and/or be exposed to wet chemical etches. Most photoresists cannot withstand these environments without flowing and thereby altering their shape and concomitantly changing masking dimensions, such as line widths and the like. It is therefore necessary to treat the photoresist in some manner to prevent flow thereof which causes such shape alteration.
In order to minimize this problem, photoresists of the type generally used are treated with deep ultraviolet light to crosslink and harden the photoresist. In the case of photoresists formed of phenolic resin, generally known by the name Novolak, which are the generally used photoresists, all commercial deep ultraviolet hardening processes use broadband spectral light sources. Wavelengths below about 250 nm. are strongly absorbed by the phenolic resin and penetrate about 1000 to 2000 angstroms to from a very thin shell on the surface of the resin. Such prior art systems require further heating of the resist to high temperatures, typically 180 degrees C. and higher, to crosslink the bulk of the resist below the small shell region since crosslinking achieved by use of such prior art ultraviolet processes takes place only at the small shell region near the surface. This further heating is accomplished either as a separate process or is accomplished in situ with a ramp bake.
The inability of the current systems to promote crosslinking throughout the resist without further heating is due to the limited penetration of most of the deep UV spectral range into the resist, coupled with the relatively low power output at the "optimum" wavelengths. Thus, the deep UV hardening process relies on the creation of a thin, highly crosslinked, hard shell formed by the shorter UV wavelengths, typically less than 290 nm. This shell then serves to contain the bulk of the resist as it is further heated beyond the flow point to high enough temperatures to initiate widespread thermal crosslinking in the bulk. As can be observed, the prior art system of minimizing flow of the photoresist during processing requires a hardening step as well as a subsequent heating step. It is desirable to eliminate the heating step in view of the economics obtained by saving a manufacturing step as well as by prevention of damage to the circuit which may be caused by the elevated temperature thereof. For example, for aluminum metal levels, baking the wafer near 200 degrees C. can lead to hillock growth. Also, baking an oxide level above 130 degrees C. has been found to reduce the adhesion of the resist to the oxide, leading to excessive undercut during oxide wet etch. Furthermore, wrinkling in the resist occurs during the subsequent high temperature bake or ramp bake. The wrinkling is directly correlated with the presence of the shorter wavelengths during the exposure, that is, use of the shorter, more strongly absorbed wavelengths can lead to wrinkling during the subsequent bake. The prior art has, on occasion, filtered out the spectral output below 240 nm. and reduced the spectral output from 240 nm. to 300 nm. to minimize this problem.
In accordance with the present invention, the above noted problems of the prior art are minimized and there is provided a procedure whereby the photoresist is hardened in a single step and at lower temperatures than are encountered in the prior art.
Briefly, the present invention provides for low temperature (less than 120 degrees C.) hardening of photoresist patterns whereas current commercial processes require additional high temperature baking of the resist to fully crosslink (harden) the resist. Such high temperatures (180 to 200 degrees C.) pose problems for some device process flows.
In the present invention, a laser of sufficient power (greater than 20 watts with output wavelength greater than about 300 nm. and preferably an excimer laser which provides a high power output at 308 nm is used which leads to crosslinking throughout the resist. A hotplate (constant temperature 120 degrees C.) is used to accelerate the crosslinking reaction, thus increasing throughput. The light beam from the laser is homogenized and expanded prior to impingement upon the photoresist. The light beam at wavelengths above about 300 nm. and preferably 308 nm. passes through the entire photoresist layer and causes crosslinking of all of the epoxy composing the photoresist, thereby minimizing photoresist movement during processing and providing more reliable results than in the prior art.